In object-oriented programming, the decorator pattern is a design pattern that allows behavior to be added to an individual object, without affecting the behavior of other objects from the same class. The decorator pattern is useful for adhering to the Single Responsibility Principle, as it allows functionality to be divided between classes with unique areas of concern; the decorator pattern is structurally nearly identical to the chain of responsibility pattern, the difference being that in a chain of responsibility one of the classes handles the request, while for the decorator, all classes handle the request. The Decorator design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change and reuse. What problems can the Decorator design pattern solve? Responsibilities should be added to an object dynamically at run-time.
A flexible alternative to subclassing for extending functionality should be provided. When using subclassing, different subclasses extend a class in different ways, but an extension can't be changed at run-time. What solution does the Decorator design pattern describe? Define Decorator objects that implement the interface of the extended object transparently by forwarding all requests to it and perform additional functionality before/after forwarding a request; this enables to work through different Decorator objects to extend the functionality of an object dynamically at run-time. See the UML class and sequence diagram below; the decorator pattern can be used to extend the functionality of a certain object statically, or in some cases at run-time, independently of other instances of the same class, provided some groundwork is done at design time. This is achieved by designing a new Decorator class; this wrapping could be achieved by the following sequence of steps: Subclass the original Component class into a Decorator class.
This pattern is designed so that multiple decorators can be stacked on top of each other, each time adding a new functionality to the overridden method. Note that decorators and the original class object share a common set of features. In the previous diagram, the operation method was available in both the decorated and undecorated versions; the decoration features are defined by an interface, mixin or class inheritance, shared by the decorators and the decorated object. In the previous example the class Component is inherited by both the ConcreteComponent and the subclasses that descend from Decorator; the decorator pattern is an alternative to subclassing. Subclassing adds behavior at compile time, the change affects all instances of the original class; this difference becomes most important when there are several independent ways of extending functionality. In some object-oriented programming languages, classes cannot be created at runtime, it is not possible to predict, at design time, what combinations of extensions will be needed.
This would mean. By contrast, decorators are objects, created at runtime, can be combined on a per-use basis; the I/O Streams implementations of both the. NET Framework incorporate the decorator pattern; as an example, consider a window in a windowing system. To allow scrolling of the window's contents, one may wish to add horizontal or vertical scrollbars to it, as appropriate. Assume windows are represented by instances of the Window interface, assume this class has no functionality for adding scrollbars. One could create a subclass ScrollingWindow that provides them, or create a ScrollingWindowDecorator that adds this functionality to existing Window objects. At this point, either solution would be fine. Now, assume one desires the ability to add borders to windows. Again, the original Window class has no support; the ScrollingWindow subclass now poses a problem, because it has created a new kind of window. If one wishes to add border support to many but not all windows, one must create subclasses WindowWithBorder and ScrollingWindowWithBorder etc.
This problem gets worse with every new window subtype to be added. For the decorator solution, we create a new BorderedWindowDecorator—at runtime, we can decorate existing windows with the ScrollingWindowDecorator or the BorderedWindowDecorator or both, as we see fit. Notice that if the functionality needs to be added to all Windows, you could modify the base class and that will do. On the other hand, sometimes it is legal, or convenient to modify the base class. Note, in the previous example, that the "SimpleWindow" and "WindowDecorator" classes implement the "Window" interface, which defines the "draw" method and the "getDescription" method, that are required in this scenario, in order to decorate a window control. A decorator makes it possible to alter behavior of an interface at run-time. Alternatively, the adapter can be used when the wrapper must respect a particular interface and must support polymorphic behavior, and
In software engineering, the module pattern is a design pattern used to implement the concept of software modules, defined by modular programming, in a programming language with incomplete direct support for the concept. This pattern can be implemented in several ways depending on the host programming language, such as the singleton design pattern, object-oriented static members in a class and procedural global functions. In Python, the pattern is built into the language, each.py file is automatically a module. The module software design pattern provides the features and syntactic structure defined by the modular programming paradigm to programming languages that have incomplete support for the concept. In software development, source code can be organized into components that accomplish a particular function or contain everything necessary to accomplish a particular task. Modular programming is one of those approaches; the concept of a "module" is not supported in many common programming language.
In order to consider that a Singleton or any group of related code implements this pattern, the following features must be supplied: A portion of the code must have global or public access and be designed for use as global/public code. Additional private or protected code can be executed by the main public code. A module must have an initializer function, equivalent to, or complementary to an object constructor method; this feature is not supported by regular namespaces. A module must have a finalizer function, equivalent to, or complementary to an object destructor method; this feature is not supported by regular namespaces. Supporting members may require initialization/finalization code, executed by the module's initializer/finalizer function. Most members are functions that perform operations on elements external to the class, provided as arguments by calling functions; such functions are "utilities", "tools" or "libraries". The semantics and syntax of each programming language may affect the implementation of this pattern.
Note that this example applies to procedural non-modular Pascal. Many Pascal dialects have namespace support, called "unit"; some dialects support initialization and finalization. If namespaces are not supported, it is recommended to give all member names a prefix related to the filename or module name in order to prevent identifier collisions. Both namespaces and modules allow to group several related entities by a single identifier, in some situations, used interchangeably; those entities can be globally accessed. The main purpose of both concepts is the same. In some scenarios a namespace requires that the global elements that compose it are initialized and finalized by a function or method call. In many programming languages, namespaces are not directly intended to support an initialization process nor a finalization process, are therefore not equivalent to modules; that limitation can be worked around in two ways. In namespaces that support global functions, a function for initialization and a function for finalization are coded directly, called directly in the main program code.
Classes are used sometimes used with namespaces. In programming languages that don't support namespaces but do support classes and objects, classes are used to substitute for namespaces; these classes are not instantiated and consist of static members. In object-oriented programming languages where namespaces are incompletely supported, the singleton pattern may be used instead of static members within a non-instantiable class; the module pattern can be implemented using a specialization of the singleton pattern. However, other design patterns may be combined, in the same class; this pattern can be used as a flyweight, or an adapter. The Module pattern can be considered a structural pattern, it manages the creation and organization of other elements, groups them as the structural pattern does. An object that applies this pattern can provide the equivalent of a namespace, providing the initialization and finalization process of a static class or a class with static members with cleaner, more concise syntax and semantics.
It supports specific cases where a object can be considered structured, procedural data. And, vice versa, migrate structured, procedural data, considered as object-oriented. Design pattern Design Patterns Singleton pattern Adapter pattern
In software engineering, dependency injection is a technique whereby one object supplies the dependencies of another object. A dependency is an object. An injection is the passing of a dependency to a dependent object; the service is made part of the client's state. Passing the service to the client, rather than allowing a client to build or find the service, is the fundamental requirement of the pattern; the intent behind dependency injection is to decouple objects to the extent that no client code has to be changed because an object it depends on needs to be changed to a different one. This permits following the Open / Closed principle. Dependency injection is one form of the broader technique of inversion of control; as with other forms of inversion of control, dependency injection supports the dependency inversion principle. The client delegates the responsibility of providing its dependencies to external code; the client is not allowed to call the injector code. This means the client code does not need to know about the injecting code, how to construct the services or which actual services it is using.
This separates the responsibilities of construction. There are three common means for a client to accept a dependency injection: setter-, interface- and constructor-based injection. Setter and constructor injection differ by when they can be used. Interface injection differs in; each requires that separate construction code takes responsibility for introducing a client and its dependencies to each other. The Dependency Injection design pattern solves problems like: How can an application or class be independent of how its objects are created? How can the way objects are created be specified in separate configuration files? How can an application support different configurations? Creating objects directly within the class that requires the objects is inflexible because it commits the class to particular objects and makes it impossible to change the instantiation independently from the class, it stops the class from being reusable if other objects are required, it makes the class hard to test because real objects can't be replaced with mock objects.
A class is no longer responsible for creating the objects it requires, it doesn't have to delegate instantiation to a factory object as in the Abstract Factory design pattern. See the UML class and sequence diagram below. Dependency injection separates the creation of a client's dependencies from the client's behavior, which allows program designs to be loosely coupled and to follow the dependency inversion and single responsibility principles, it directly contrasts with the service locator pattern, which allows clients to know about the system they use to find dependencies. An injection, the basic unit of dependency injection, is not a custom mechanism, it works in the same way. Referring to "parameter passing" as an injection carries the added implication that it's being done to isolate the client from details. An injection is about what is in control of the passing and is independent of how the passing is accomplished, whether by passing a reference or a value. Dependency injection involves four roles: the service object to be used the client object, depending on the service it uses the interfaces that define how the client may use the services the injector, responsible for constructing the services and injecting them into the clientAny object that may be used can be considered a service.
Any object that uses other objects can be considered a client. The names have nothing to do with what the objects are for and everything to do with the role the objects play in any one injection; the interfaces are the types. An issue is, they may be interface types implemented by the services but may be abstract classes or the concrete services themselves, though this last would violate DIP and sacrifice the dynamic decoupling that enables testing. It's only required that the client does not know which they are and therefore never treats them as concrete, say by constructing or extending them; the client should have no concrete knowledge of the specific implementation of its dependencies. It should only know the interface's name and API; as a result, the client won't need to change if what is behind the interface changes. However, if the interface is refactored from being a class to an interface type the client will need to be recompiled; this is significant if services are published separately.
This unfortunate coupling is one. The injector introduces the services into the client, it constructs the client. An injector may connect together a complex object graph by treating an object like a client and as a service for another client; the injector may be many objects working together but may not be the client. The injector may be referred to by other names such as: assembler, container, builder, construction code, or main. Dependency injection can be applied as a discipline, one that asks that all objects separate construction and behavior. Relying on a DI framework to perform construction can lead to forbidding the use of the new keyword, or, less only allowing direct constr
In computer programming, flyweight is a software design pattern. A flyweight is an object that minimizes memory usage by sharing as much data as possible with other similar objects; some parts of the object state can be shared, it is common practice to hold them in external data structures and pass them to the objects temporarily when they are used. A classic example usage of the flyweight pattern is the data structures for graphical representation of characters in a word processor, it might be desirable to have, for each character in a document, a glyph object containing its font outline, font metrics, other formatting data, but this would amount to hundreds or thousands of bytes for each character. Instead, for every character there might be a reference to a flyweight glyph object shared by every instance of the same character in the document. Another example is string interning. In other contexts the idea of sharing identical data structures is called hash consing; the Flyweight design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change and reuse.
What problems can the Flyweight design pattern solve? Large numbers of objects should be supported efficiently. Creating large numbers of objects should be avoided; when representing large text documents, for example, creating an object for each character in the document would result in a huge number of objects that could not be processed efficiently. What solution does the Flyweight design pattern describe? Define Flyweight objects that store intrinsic state that can be shared and provide an interface through which extrinsic state can be passed in; this enables clients to reuse Flyweight objects and pass in extrinsic state when they invoke a Flyweight operation. This reduces the number of physically created objects. Intrinsic state therefore can be shared. Extrinsic state therefore can not be shared and must be passed in. See the UML class and sequence diagram below. According to the textbook Design Patterns: Elements of Reusable Object-Oriented Software, the flyweight pattern was first coined and extensively explored by Paul Calder and Mark Linton in 1990 to efficiently handle glyph information in a WYSIWYG document editor, although similar techniques were used in other systems, e.g. an application framework by Weinand et al..
In the above UML class diagram, the Client class refers to the FlyweightFactory class to create/share Flyweight objects and to the Flyweight interface to perform an operation by passing in extrinsic state. The Flyweight1 class implements the Flyweight interface and stores intrinsic state that can be shared; the sequence diagram shows the run-time interactions: The Client object calls getFlyweight on the FlyweightFactory that creates and returns a Flyweight1 object. After calling operation on the returned Flyweight1 object, the Client again calls getFlyweight on the FlyweightFactory, which now shares and returns the existing Flyweight1 object. To enable safe sharing, between clients and threads, Flyweight objects must be immutable. Flyweight objects are by definition value objects; the identity of the object instance is of no consequence therefore two Flyweight instances of the same value are considered equal. Example in C#: Special consideration must be made in scenarios where Flyweight objects are created on multiple threads.
If the list of values is finite and known in advance the Flyweights can be instantiated ahead of time and retrieved from a container on multiple threads with no contention. If Flyweights are instantiated on multiple threads there are two options: Make Flyweight instantiation single threaded thus introducing contention and ensuring one instance per value. Allow concurrent threads to create multiple Flyweight instances thus eliminating contention and allowing multiple instances per value; this option is only viable. Flyweight allows you to share bulky data. In other words, if you think that same data is repeating for every object, you can use this pattern to point to the single object and hence can save space. Here the FlyweightPointer creates a static member Company, used for every object of MyObject; the execution of this code will give the following: Attributes can be defined at the class-level instead of only for instances in Python because classes are first-class objects in the language—meaning there are no restrictions on their use as they are the same as any other object.
New-style class instances store instance data in a special attribute dictionary instance.__dict__. By default, accessed attributes are first looked-up in this __dict__, fallback to the instance's class attributes next. In this way, a class can be a kind of Flyweight container for its instances. Although Python classes are mutable by default, immutability can be emulated by overriding the class's __setattr__ method so that it disallows changes to any Flyweight attributes. Output Serving Cappuchino
In software engineering, creational design patterns are design patterns that deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. The basic form of object creation could result in design problems or in added complexity to the design. Creational design patterns solve this problem by somehow controlling this object creation. Creational design patterns are composed of two dominant ideas. One is encapsulating knowledge. Another is hiding how instances of these concrete classes are combined. Creational design patterns are further categorized into Object-creational patterns and Class-creational patterns, where Object-creational patterns deal with Object creation and Class-creational patterns deal with Class-instantiation. In greater details, Object-creational patterns defer part of its object creation to another object, while Class-creational patterns defer its object creation to subclasses. Five well-known design patterns that are parts of creational patterns are the Abstract factory pattern, which provides an interface for creating related or dependent objects without specifying the objects' concrete classes.
Builder pattern, which separates the construction of a complex object from its representation so that the same construction process can create different representations. Factory method pattern, which allows a class to defer instantiation to subclasses. Prototype pattern, which specifies the kind of object to create using a prototypical instance, creates new objects by cloning this prototype. Singleton pattern, which ensures that a class only has one instance, provides a global point of access to it; the creational patterns aim to separate a system from how its objects are created and represented. They increase the system's flexibility in terms of the what, who and when of object creation.. As modern software engineering depends more on object composition than class inheritance, emphasis shifts away from hard-coding behaviors toward defining a smaller set of basic behaviors that can be composed into more complex ones. Hard-coding behaviors are inflexible because they require overriding or re-implementing the whole thing in order to change parts of the design.
Additionally, hard-coding makes it difficult to keep track of errors. For these reasons, creational patterns are more useful than hard-coding behaviors. Creational patterns make design become more flexible, they provide different ways to remove explicit references in the concrete classes from the code that needs to instantiate them. In other words, they create independency for classes. Consider applying creational patterns when: A system should be independent of how its objects and products are created. A set of related objects is designed to be used together. Hiding the implementations of a class library or product, revealing only their interfaces. Constructing different representation of independent complex objects. A class wants its subclass to implement the object; the class instantiations are specified at run-time. There must be a single instance and client can access this instance at all times. Instance should be extensible without being modified. Below is a simple class diagram. Note that different creational patterns require additional and different participated classes.
Participants: Creator: Declares object interface. Returns object. ConcreteCreator: Implements object's interface; some examples of creational design patterns include: Abstract Factory pattern: a class requests the objects it requires from a factory object instead of creating the objects directly Factory method pattern: centralize creation of an object of a specific type choosing one of several implementations Builder pattern: separate the construction of a complex object from its representation so that the same construction process can create different representations Dependency Injection pattern: a class accepts the objects it requires from an injector instead of creating the objects directly Lazy initialization pattern: tactic of delaying the creation of an object, the calculation of a value, or some other expensive process until the first time it is needed Object pool pattern: avoid expensive acquisition and release of resources by recycling objects that are no longer in use Prototype pattern: used when the type of objects to create is determined by a prototypical instance, cloned to produce new objects Singleton pattern: restrict instantiation of a class to one object Constructor Behavioral pattern Concurrency pattern Structural pattern
The state pattern is a behavioral software design pattern that allows an object to alter its behavior when its internal state changes. This pattern is close to the concept of finite-state machines; the state pattern can be interpreted as a strategy pattern, able to switch a strategy through invocations of methods defined in the pattern's interface. The state pattern is used in computer programming to encapsulate varying behavior for the same object, based on its internal state; this can be a cleaner way for an object to change its behavior at runtime without resorting to conditional statements and thus improve maintainability. The state design pattern is one of the twenty-three design patterns designed by the Gang of Four that describe how to solve recurring design problems; such problems cover the design of flexible and reusable object-oriented software, such as objects that are easy to implement, change and reuse. The state pattern is set to solve two main problems: An object should change its behavior when its internal state changes.
State-specific behavior should be defined independently. That is, adding new states should not affect the behavior of existing states. Implementing state-specific behavior directly within a class is inflexible because it commits the class to a particular behavior and makes it impossible to add a new state or change the behavior of an existing state independently from the class. In this, the pattern describes two solutions: Define separate objects that encapsulate state-specific behavior for each state; that is, define an interface for performing state-specific behavior, define classes that implement the interface for each state. A class delegates state-specific behavior to its current state object instead of implementing state-specific behavior directly; this makes a class independent of. New states can be added by defining new state classes. A class can change its behavior at run-time by changing its current state object. In the above Unified Modeling Language class diagram, the Context class doesn't implement state-specific behavior directly.
Instead, Context refers to the State interface for performing state-specific behavior, which makes Context independent of how state-specific behavior is implemented. The State1 and State2 classes implement the State interface, that is, implement the state-specific behavior for each state; the UML sequence diagram shows the run-time interactions: The Context object delegates state-specific behavior to different State objects. First, Context calls operation on its current state object, which performs the operation and calls setState on Context to change context's current state to State2; the next time, Context again calls operation on its current state object, which performs the operation and changes context's current state to State1. The state interface and two implementations; the state's method is able to change its state. The context class has a state variable that it instantiates in an initial state, in this case LowerCaseState. In its method, it uses the corresponding methods of the state object.
The demonstration below shows the usage: With the above code, the output of main from StateDemo is: monday TUESDAY WEDNESDAY thursday FRIDAY SATURDAY sunday
In object-oriented design, the chain-of-responsibility pattern is a design pattern consisting of a source of command objects and a series of processing objects. Each processing object contains logic. A mechanism exists for adding new processing objects to the end of this chain. Thus, the chain of responsibility is an object oriented version of the if... else if... else if....... Else... endif idiom, with the benefit that the condition–action blocks can be dynamically rearranged and reconfigured at runtime. In a variation of the standard chain-of-responsibility model, some handlers may act as dispatchers, capable of sending commands out in a variety of directions, forming a tree of responsibility. In some cases, this can occur recursively, with processing objects calling higher-up processing objects with commands that attempt to solve some smaller part of the problem. An XML interpreter might work in this manner; this pattern promotes the idea of loose coupling. The chain-of-responsibility pattern is structurally nearly identical to the decorator pattern, the difference being that for the decorator, all classes handle the request, while for the chain of responsibility one of the classes in the chain handles the request.
The Chain of Responsibility design pattern is one of the twenty-three well-known GoF design patterns that describe common solutions to recurring design problems when designing flexible and reusable object-oriented software, that is, objects that are easier to implement, change and reuse. What problems can the Chain of Responsibility design pattern solve? Coupling the sender of a request to its receiver should be avoided, it should be possible. Implementing a request directly within the class that sends the request is inflexible because it couples the class to a particular receiver and makes it impossible to support multiple receivers. What solution does the Chain of Responsibility design pattern describe? Define a chain of receiver objects having the responsibility, depending on run-time conditions, to either handle a request or forward it to the next receiver on the chain; this enables to send a request to a chain of receivers without having to know which one handles the request. The request gets passed along the chain.
The sender of a request is no longer coupled to a particular receiver. See the UML class and sequence diagram below. In the above UML class diagram, the Sender class doesn't refer to a particular receiver class directly. Instead, Sender refers to the Handler interface for handling a request, which makes the Sender independent of which receiver handles the request; the Receiver1, Receiver2, Receiver3 classes implement the Handler interface by either handling or forwarding a request. The UML sequence diagram shows the run-time interactions: In this example, the Sender object calls handleRequest on the receiver1 object; the receiver1 forwards the request to receiver2, which in turn forwards the request to receiver3, which handles the request. Below is an example of this pattern in Java. A logger is created using a chain of loggers, each one configured with different log levels; this C# examples uses the logger application to select different sources based on the log level. Writing to console: Order record retrieved.
Writing to console: Customer Address details missing in Branch DataBase. Writing to Log File: Customer Address details missing in Branch DataBase. Writing to console: Customer Address details missing in Organization DataBase. Writing to Log File: Customer Address details missing in Organization DataBase. Writing to console: Unable to Process Order ORD1 Dated D1 For Customer C1. Sending via email: Unable to Process Order ORD1 Dated D1 For Customer C1. Writing to console: Order Dispatched. Sending via email: Order Dispatched; the Cocoa and Cocoa Touch frameworks, used for OS X and iOS applications actively use the chain-of-responsibility pattern for handling events. Objects that participate in the chain are called responder objects, inheriting from the NSResponder /UIResponder class. All view objects, view controller objects, window objects, the application object are responder objects; when a view receives an event which it can't handle, it dispatches it to its superview until it reaches the view controller or window object.
If the window can't handle the event, the event is dispatched to the application object, the last object in the chain. For example: On OS X, moving a textured window with the mouse can be done from any location, unless on that location there's a view which handles dragging events, like slider controls. If no such view is there, dragging events are sent up the chain to the window which does handle the dragging event. On iOS, it's typical to handle view events in the view controller which manages the view hierarchy, instead of subclassing the view itself. Since a view controller lies in the responder chain after all of its managed subviews, it can intercept any view events and handle them. Software design pattern Single responsibility principle